Use this respiratory calculation when pressures, gas fractions, flows, and volumes use the stated units and were measured under compatible conditions. It can reproduce a defined mechanics or gas-exchange quantity.

Peak inspiratory pressure

A finite measured or assumed value, expressed in cmH2O.

Pause plateau pressure

A finite measured or assumed value, expressed in cmH2O.

Inspiratory flow

A finite measured or assumed value, expressed in L/s.

The implementation follows this calculation rule: Compute total airway resistance as the difference between peak inspiratory pressure and pause plateau pressure divided by inspiratory flow.

Total airway resistance

The calculated total airway resistance in cmH2O/L/s.

These simplified equations do not replace pulmonary-function testing or clinical assessment. Barometric pressure, temperature, water-vapour pressure, respiratory quotient, leaks, ventilator timing, spontaneous effort, and measurement site can change the result. Ratios and percentages become unstable when their denominator is small, uncertain, or defined differently between samples.

Interpret the result with arterial or end-tidal gases, ventilator waveforms, oxygen saturation, and serial measurements as appropriate. Directly measured mechanics or gas exchange is preferable when the simplifying assumptions do not hold.

• Airway Resistance

  Compute airway resistance as peak pressure minus plateau pressure divided by inspiratory flow.

• Dynamic Compliance

  Compute dynamic respiratory compliance as tidal volume divided by peak inspiratory pressure minus PEEP.

• Total Compliance

  Compute total respiratory system compliance as exhaled tidal volume divided by the driving pressure between plateau pressure and total PEEP.